A helmet is not an accessory, it a life saving gear. You must therefore spend considerable time in finding out how to buy a motorcycle helmet. Some of the points that you need to check are:
1. DOT Certification: The helmet that you buy must have a Department of Transportation certification. This is a guarantee that the helmet meets the basic safety requirements.
2.

Retention: Do a roll off test before you buy a helmet. This test is simple. All that you need to do is to secure the straps, grab the rear lip of the helmet and try to roll it forward off your head. Don't stop if it hurts. A helmet should stay on your head at the moment of impact.

If the helmet comes off, then don’t buy it. There is no point having a fractured head and an intact helmet lying a few feet away.
3. Fit: The fit decides the retention factor of a helmet. The fit should be snug and the helmet should stay stable. A full-face helmet should grip your cheeks and jaw as well as the top and sides of your head. When you move the helmet your skin must move as well.
4. Comfort: Comfort is a sum total of many things: the fit, the feel, the weight, the soundproofing. When trying on a helmet keep it on for at least 20 minutes.

Then check to see what are the areas of discomfort.
5. Coverage: A full helmet is better than a half helmet and a full-face helmet (with a chin bar) gives more protection than an open-face style.
6. Face shield: If you are buying a helmet with a face shield go for Snell-certified helmets. The shield should be easy to operate and stay in position when raised. Also check for optical distortions. They can be dangerous.
7. Convenience features: Along with an extra shield and a quick-change face shield mechanism, a storage bag is a useful feature to look for.

You can use it to carry helmets.
8. Comfort features: Vents, padded straps and, interchangeable padding shapes to fit your head shape are comfort features. But remember vents can cause wind noise and can also get damaged easily.
9. Appearance: Appearance is largely a personal choice. The only advantage that a florescent helmet has is that it will stand out more during the day.
10. Cost: Go for the helmet that meets all the criteria outlined above, even if it is cheaper than other helmets. Price is not always an absolute indicator of quality.